Cancer Immune Therapy Edited by G. Stuhler and P. Walden ...

270 13 Applications of CpG Motifs from Bacterial DNA in Cancer Immunotherapy
is an effective vaccine adjuvant [18]. In fact, several human clinical trials in cancer
were performed with various formulations of poly(rI 7 rC), but the clinical activity was
disappointing in comparison to the substantial toxicity [19, 20]Ç The immune activation
triggered by this polynucleotide is thought to be mediated by the double-stranded
RNA-dependent protein kinase, PKR [21].
Another polynucleotide which has also been shown to have immune stimulatory effects
is poly(A 7 U). Like poly(rI 7rC), poly(A) 7poly(U) has been shown to have immune
stimulatory activity in animal models [22]. Although it has some similar properties
to poly(rI 7 rC) in terms of its ability to promote production of IFNs and activate
NK cells, poly(A 7 U) has been proposed to be less toxic. Poly(A 7U) has also
been shown to have adjuvant effects when administered in combination with hepatitis
B surface antigen to mice [23]. In human clinical trials, poly(A 7 U) has shown
some encouraging signs of efficacy in patients with breast cancer [24, 25]. Poly(A 7U)
has also been reported to have substantial therapeutic activity without significant
side effects when used as an immune modulator for the treatment of chronic active
hepatitis B [26]. NK cell activation has also been reported to be triggered by
poly(dG7dC) [27]. DNA containing runs of consecutive guanines, termed poly(G) sequences
or G-quartets, can induce B cell proliferation [28]. Fragments of doublestranded
DNA or RNA as short as 25 bp have been reported to induce non-immune
cells to induce or activate STAT1, STAT3, NF-kB and mitogen-activated protein kinases
(MAPKs), and to express MHC and co-stimulatory molecules in a sequence-independent
manner that required the DNA or RNA to be introduced into the cell cytoplasm,
which could possibly occur during a viral infection [29]. In contrast to bDNA,
which is active in either double- or single-stranded form, vertebrate DNA is only active
in the double-stranded form [29]. Introduction of host DNA into the cytoplasm
of dendritic cells (DCs) induces them to mature, with enhanced functional activity
[30]. It is unclear whether these different immune stimulatory polynucleotides exert
their effects through the same or different mechanisms.
13.2
CpG Motifs in bDNA Explain its Immune Stimulatory Activity
As reviewed above, Tokunaga et al. and Pisetsky et al. demonstrated that bDNA activated
NK and B cells, but that vertebrate DNA was inactive. Although Tokunaga proposed
that palindromes in the bDNA were responsible for its immune stimulatory
activity [9], further studies demonstrated that a sufficient sequence element was a
CpG dinucleotide in particular base contexts, termed ªCpG motifsº [reviewed in 31].
Elimination of the CpG dinucleotides from ODN abolished their stimulatory activities
[32]. The role of the CpG motif as the active element in bDNA provides a new
insight into the old observation that vertebrate and bDNAs differ markedly in their
CpG content and methylation [33]. Bacterial DNA generally contains the expected
frequency of about one CpG dinucleotide per 16 bases. However, CpG dinucleotides
are known to be markedly suppressed in vertebrate genomes: they occur only about
a quarter as frequently as would be predicted if base utilization was random [33].